Needleless injectors, such as those described in U.S. Pat. No. 5,599,302 issued to Lilley et al., U.S. Pat. No. 5,062,830 to Dunlap, and U.S. Pat. No. 4,790,824 to Morrow et al., must eject medicament at a pressure (P.sub.piercing) that is sufficient to pierce the skin so that the medicament can be delivered to the desired area, usually the subcutaneous region. Ordinarily, P.sub.piercing is approximately 4000 psi. The need to achieve such a high pressure has imposed certain design limitations that affect many of the operational aspects of needleless injectors. One such aspect is the manner in which needleless injectors are filled with medicament.
For conventional needle-containing syringes, filling the syringe with medicament requires only simple "pull-push" motions. Specifically, the plunger of the syringe is pulled back with the needle inserted in the medicament vial to fill the syringe with the desired amount of medicament. After the needle is removed from the vial, the plunger is pushed forward to prime the syringe, i.e., remove any air bubbles. In contrast, filling needleless injectors with medicament is a laborious and lengthy multi-step procedure. For example, with the Medi-Jector Choice.TM. injector available from Medi-Ject Corporation of Minneapolis, Minn., the body of the injector is repeatedly twisted in a first direction to ready the injector for medicament delivery. Next, the user attaches the nozzle of the injector to the medicament vial and repeatedly twists the body in a second direction until the desired amount of medicament is drawn into the nozzle chamber. Finally, the user twists the body in the first direction to prime the injector, i.e., remove any air bubbles from the nozzle chamber. As the Medi-Jector Choice.TM. injector, like any conventional needleless injector, uses a high force energy mechanism (i.e., a coil spring), twisting the body of the injector can be difficult.
Advances in needleless injector technology have made the use of lower force energy mechanisms feasible. The use of lower force energy mechanisms has reduced the design constraints on needleless injectors. Lower force energy mechanisms would also be useful in intradermal applications such as vaccine, specifically DNA vaccines in which a high force energy mechanism could disrupt the molecular structure. Such use is disclosed in "Intradermal DNA Immunization by Using Jet-Injectors in Mice and Monkeys," Vaccine, 17:628-38, February 1999.
Thus, for both high force and low force injectors, there still exists a need for a needleless injector using an improved loading mechanism to fill the injector with medicament.